Thumper Club Forum
Technical => Bike Problems/Questions => Topic started by: Steve Lake on March 26, 2026, 11:35:33 AM
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all was going well, took barrel + 50thou oversize piston to the engineers last week.
they rang me up today and asked for the factory recommended clearances.
being a race prep works (mainly 1275 S BMC A series engines & BMW JCW cooper S's) they are understandably a bit fussy about these things.
and i haven't a clue, my workshop manual doesn't help much so i'm reaching out to the club experts.....
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It's tricky as it depends on piston material, lubrication, block / liner material, air / water cooled etc. Can you contact the piston manufacturer? For my old clunkers, I generally use much bigger clearances than you would, eg .005" for a 79mm piston. I'd expect there to be quite a difference between A series and BMW mini engines.
My philosophy is always to err on the generous side as I'd rather put up with a bit of piston slap and oil consumption rather than risk seizure and spend hundreds of miles running in.
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pretty much my feelings Ian ....
but this afternoon whilst rummaging through copious SRX literature that i've amassed over the years i found this....
so have forwarded it to the engineer.
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Hard to read upside down but those are much tighter than I would use. It is a modern engine however.....